Microsoft's 16-month browser ballot screw-up in the European Union cost Mozilla an estimated 8.8 million downloads of its Firefox browser, the open-source vendor's head lawyer said Tuesday.

Microsoft faces fines that could reach into the billions for omitting a browser choice screen it was supposed to show European users of Windows 7. The blunder began in late February 2011, when Microsoft shipped Windows 7 Service Pack 1 (SP1), and ran through early July 2012, when authorities notified the company of the oversight.

According to Harvey Anderson, Mozilla's general counsel, Firefox's daily download average fell 63% from approximately 54,000 to a low of 20,000 before Microsoft rolled out a fix for the snafu last summer. After Microsoft updated EU users' Windows 7 PCs with a patch to restore the ballot screen, Firefox downloads increased 150% to an average of approximately 50,000, Anderson claimed.

During the more than 16 months when the ballot was not offered EU users, Mozilla "lost" between 6 and 9 million downloads. Anderson included a graph in his blog post, however, that put the impact at a more specific 8.8 million Firefox downloads.

Last week, the European Competition Commission -- the EU government's antitrust agency -- served Microsoft with formal charges for failing to display the required browser choice screen to systems upgraded to Windows 7 SP1. Previously, the Commission had said that some 28 million users in the EU had been shortchanged.

The browser ballot was the result of a deal Microsoft struck with the Commission in late 2009 after officials launched an investigation triggered by a complaint from Norwegian browser maker Opera Software. Opera accused Microsoft of manipulating the battle for browser share by tying Internet Explorer (IE) to Windows.

The settlement required Microsoft to display a screen in Windows that provided download links to other browsers, including Apple's Safari, Google's Chrome, Mozilla's Firefox and Opera Software's Opera. Safari has since been dropped from the deal, as Apple has halted development of the Windows version.

Microsoft claimed that the omission was simply a "technical error," then apologized and quickly created an update to address the problem.

However, Microsoft blew a chance to correct its flub -- and save it from what may end up being massive fines -- just weeks after the release of Windows 7 SP1, when it ignored a tip from a customer who had noticed the missing browser ballot and reported that to the company's support staff.

Mozilla's decline in the share of the European market sharply accelerated during the stretch when Microsoft was not showing Windows 7 users the ballot.

Mozilla claims that Microsoft's browser ballot omission in the EU resulted in 8.8 million fewer downloads of Firefox than would otherwise likely have occurred. (Image: Mozilla.)